1. Field of the Invention
The present invention relates to an optical system, and more particularly, to an optical system corrected for aberrations, which is suitable for use in an objective lens of a 35 mm single-lens reflex camera, an electronic still camera, a video camera or the like.
2. Description of the Related Art
In a popular type of objective lens, sufficient performance cannot be obtained unless not only spherical aberration, coma and astigmatism, but also axial chromatic aberration and chromatic aberration of magnification are corrected well.
In conventional aberration correction using only a refracting optical element, chromatic aberration is almost always determined in a relatively early design stage in which the power and the material of the refracting optical element are determined. For example, axial chromatic aberration is overcorrected in the short wavelength region of the visible range, insufficiently corrected in the intermediate region, and overcorrected again in the long wavelength region, that is, the axial chromatic aberration has a tendency of what is called achromat.
As a well-known method of correcting axial chromatic aberration with respect to a wide range of wavelengths, fluorite or the like, which has a low refractive index and a low dispersive power and which is anomalously dispersive, is used in a convex lens, whereby residual chromatic aberration is corrected relatively well. However, such an anomalous dispersion glass is extremely expensive.
Furthermore, since the above-discussed glass has a relatively low refractive index of about 1.5, the use thereof in optical systems other than an optical system having a longer focal length than a medium-telephoto system, in which the amount of change in the field angle owing to aberration is small, makes it extremely difficult to keep aberrations in balance.
For example, in the case in which this glass material having low refractive index is used in a Gaussian optical system, which is often used as a standard lens for a 35 mm single-lens reflex camera, spherical aberration, astigmatism and field curvature cannot be kept in balance, and, in particular, spherical aberration becomes worse in the third-order region and the high-order region. Although spherical aberration in the high-order region can be reduced to zero by balancing the third-order spherical aberration coefficients and the high-order spherical aberration coefficients, since it is difficult to decrease the absolute values of the third-order and high-order spherical aberration coefficients, zonal spherical aberration remains, as it is not corrected sufficiently. This problem can be solved by using an anomalous dispersion material having a high refractive index and a high dispersive power, but, regrettably, such a material does not exist.
In order to correct axial chromatic aberration for a wide range of wavelengths without using an anomalous dispersion glass such as fluorite, as is well known, a refracting optical element and a diffracting optical element are used in combination. Since the diffracting optical element has a negative Abbe number, it can correct chromatic aberration well in combination with the refracting optical element.
Japanese Laid-Open Patent Application No. 6-324262 discloses a typical photographic optical system which corrects chromatic aberration by using a refracting optical element and a diffracting optical element in combination.
In this publication, a diffracting optical element is placed in a telephoto optical system having a field angle of about 14.degree. (the focal length thereof is about 180 mm in a 35 mm single-lens reflex camera), thereby correcting axial chromatic aberration.
As a medium-telephoto lens in which the change in field angle owing to aberration is small, this optical system is corrected relatively well for aberrations including axial chromatic aberration. However, the F-number thereof is about 2.8, that is, the brightness is low. It is impossible to increase the aperture so that the F-number is about 1.4 and to obtain a wide field angle without changing the lens layout.